4 MANUAL OF ANIMAL BIOLOGY 



A simple way to get an idea of the cellular organization, 

 which is characteristic of protoplasm, is to make a microscopic 

 examination of a small piece of epidermis shed from the skin of the 

 Frog. A thin layer of this material forms the outer surface of the 

 skin all over the body. Such an examination will reveal the fact 

 that the skin epidermis is not a homogeneous body of material, 

 but, on the contrary, is an aggregate of great numbers of micro- 

 scopic cells which, generally, are in contact with adjacent cells on 

 all sides, thus forming a tissue. A similar examination of the 

 epidermis from our own bodies or of the various tissues from any 

 multicellular organism shows that they are built up from cellular 

 units having the same general organization as those present in the 

 epidermis of the Frog. (W. f. 2.) 



A clear conception should be obtained of the fact that cells are 

 solids, having length, breadth, and thickness, and not merely 

 flat surfaces as they appear to be when a section of some tissue is 

 examined under the microscope. The three dimensions of cells 

 can be obtained from a study of some suitable tissue in which the 

 cells are regularly arranged and which has been sectioned in two 

 different planes which lie perpendicular to each other. Thus we 

 may compare the shape of the cells as seen in a longitudinal sec- 

 tion with that found in a transverse, or vertical, section. In one 

 of these planes, the cells will be seen from a side view, which will 

 give the length and breadth, while in the other plane, an end 

 view will be seen from which the third dimension, thickness, may 

 be obtained. (W. f. 5.) 



A crude comparison is sometimes made between the cells which 

 compose a living organism and the bricks as the structural elements 

 in a building. From the structural standpoint the analogy is, 

 perhaps, a fitting one, as is well-shown by the microscopic examina- 

 tion of various tissues indicated above. From another, and much 

 more important viewpoint, however, the comparison is meaningless, 

 for the bricks are inert masses of lifeless clay which are unable to 

 play any part in the activities of the building of which each consti- 

 tutes a structural unit, while, on the other hand, every one of the 

 cells which go to make up the living organism is a dynamic, proto- 

 plasmic unit endowed with the principle we call life and continually 

 performing its share of the many functions which are essential to 

 the maintenance of life in the organism. 



